Use a Sprint Galaxy S III on Virgin Mobile

This may be a stupid question, but a friend of mine recently switched to Sprint rather than using Virgin Mobile and got a Galaxy S III on Sprint. Since then, he has had constant issues with Sprint in terms of billing and wants to go back to just using Virgin Mobile with pre-paid. Anyway, I was thinking, since he already has the Galaxy S III and it's on Sprint (therefore, it has the parts to work on their towers) could I root his phone and make it usable on Virgin Mobile via a custom ROM or something else? Any thoughts or advice would be greatly appreciated.

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